Vlf and elf spectrometer

ABSTRACT

Apparatus for detecting and recording an electromagnetic radiation spectrum in the VLF and ELF range including a ferromagnetic cored antenna in a variably tuned circuit, a drive motor for sweeping the tuning and actuating a sweep marker, a time reference, and a voice and data amplifier. The output signals of these devices are fed into a multichannel recorder. A plurality of such variably tuned circuits may be used.

mag-5 2 UR 3,638,1 2

15] 3,638,112 Lasater et al. [451 Jan. 25, 1972 [54] VLF AND ELF SPECTROMETER FOREIGN PATENTS OR APPLICATIONS 72 Inventors; James Lasmer; David woodbridge, 746,985 3/1956 Great Britain ..343/787 both f Danasv Tex 787,653 12/1957 Great Britain ..343/1 15 [73] Assignee: International Space Corporation OTHER PUBLICATIONS [22] Filed: July 26, 1963 Ogayashi Vol. 64D, No. 1, 1960, pp. 41-48.

Samson, Vol. 64D, No. 1, 1960, pp. 37 40. Watts et al., Vol. 67D, No. 5, Oct. 1963, pp. 569- 579 (received May 8, 1963).

[21] AppI.No.: 297,803

[52] US. Cl ..324/77, 324/57, 343/788 511 Int. Cl. ....G0lr 23/18, HOlq 7/08 Primary Examiner-Richard A Farley 531 Field of Search ..343/787, 788, 199, 113; Assistant Examiner-Richard Berger [57] ABSTRACT [56] Reterences cued Apparatus for detecting and recording an electromagnetic UNITED STATES PATENTS radiation spectrum in the VLF and ELF range including a ferromagnetic cored antenna in a variably tuned circuit, a drive 3,040,315 6/1962 Kramer ..343/101 motor for sweeping the tuning and actuating a sweep marker,

1,523,798 1/1925 Benson et al .343/788 X a time reference, and a voice and data amplifier. The output 2,931,974 4/1960 McLaughlin et al..

.....324/8 signals of these devices are fed into a multichannel recorder.

3,149,278 9/1964 Cartier et al ..324/7 A plurality of such variably tuned circuits may be used.

9 Claims, 4 Drawing Figures 27 37a 18 31 DRIVE 33 i 1 SWEEP AMPLIFIER MARKER AMPLIFIER Q 55 i3 1 I 11 11 s 1 wwv VOICE AND RECEIVER TAPE RECORDER AE PATEMED JANZS 1972 SHEEI 2 0F 2 mmomOwmm wad;

mwamowm 3 ill w vll'L IILI mmljasz VLF AND ELF SPECTROMETER The present invention relates to methods and apparatus for obtaining VLF (very low frequency) and ELF (extra low frequency) electromagnetic radiation spectrum from plasma and particularly to methods and apparatus for analyzing the characteristics of plasma by the detection and observation of the very low frequency and extra low frequency radiation emitted.

One of the principal properties of any plasma is. the presence of substantial numbers of electrons and positive ions. Since the plasma particles are in constant motion, the positions of the particles are a function of time. If an electron passes through the field of a nucleus or atom, it is deflected. This deflection constitutes an acceleration (or deceleration) of a charge, and according to the classical theory of electrodynamics, an accelerated (or decelerated) charge must necessarily radiate energy. The interaction of an electron with a nucleus or atom is only one example of radiated collisions. Collisions emitting radiation may involve either similar or dissimilar particles, such as,

a. two similar heavy particles b. two dissimilar heavy particles c. two dissimilar particles, one heavy and one light The first case (a) corresponds to a collision between either two ions or two neutral (excited) atoms, the second case (b) corresponds to a collision between an ion and a neutral atom, and the third case encompasses three types of collisions. The types of collision involved in the last case (c) are:

1. radiated capture of an electron by an ion 2. radiated capture of an electron by a neutral atom 3. radiated transition of an ion or an atom The last type (3) of radiation is called Bremsstrahlung or breaking radiation," and it probably is the predominant collision-type radiation.

From classical considerations, the various radiation mechanisms that are expected to be encountered with a dynamic plasma include:

a. thermal (or black-bodied) radiation b. Bremsstrahlung radiation c. gyromagnetic radiation d. Cherenkov radiation e. Excitation radiation Thermal or black-bodied radiation is emitted when the radiation is in thermodynamic equilibrium with a medium. True thermal radiation is admitted by an ideal radiator; however, an ideal radiator is seldom found in nature. Bremsstrahlung radiation is emitted when the radiation is not in thermodynamic equilibrium with a medium, and is caused by the breaking influence of nuclei upon fast electrons. Gyromagnetic radiation is emitted when charged particles rotate around the magnetic lines of force. Cherenkov radiation is emitted by a high-energy charge particle moving in a medium with an index of refraction considerably greater than unity. It is caused by the effect of the difference between the high velocity of the particle which may be close to that of light in a vacuum, the lower velocity of its associated electric and magnetic fields. The electric and magnetic fields possess a velocity equal to that of light in,a vacuum divided by the index of refraction of a medium. Excitation radiation is emitted when an electron moves from an outer to an inner shell of an atom or molecule. This type of radiation will have either line or broad spectral distribution. Not all of these types of radiations will be encountered with all plasmas. The applicable radiation mechanism depends upon the characteristics of the particular plasma system. The preceding five types of emissions are those to be anticipated from classical considerations. However, there are two additional radiation mechanisms that are particularly important for plasmas. These are nuclear magnetic resonance radiation and plasma oscillations. Nuclear magnetic resonance radiation is a consequence of changes in the orientation of the nuclear spin vector in a magnetic field. Plasma oscillation radiation arises from coherence of the plasma. These latter two mechanisms have considerable potential for diagnostic purposes.

Of the seven aforementioned types of radiation, all except excitation radiation emit electromagnetic radiation in the very low frequency or extra low frequency bands. The advancement of plasma physics has long been hampered by the lack of methods and apparatus for the detection and spectral analysis of radiation from plasma in the very low frequency and extra low frequency bands which are essentially free from mancreated radiation and which contain emissions created by most of the presently known plasma radiation mechanisms. The present invention provides apparatus and methods for the detection and spectral analysis of plasma which will provide for the further study and analysis of various plasmas to determine their characteristics and the radiation spectrum resulting from those characteristics.

In addition, the present invention opens a completely new field for the detection of events which create plasmas whose spectrum can be identified. Such events are the firing of missiles, the reentry of bodies and warheads into the earths atmosphere, and nuclear detonations.

Therefore, it is an object of the present invention to provide new and improved apparatus for the detection and spectrum analysis of electromagnetic radiation produced by plasma mechanisms.

Another object is to provide methods and apparatus for the detection of missiles, the reentry of warheads and other objects into the earths atmosphere, and nuclear detonations.

A further object is to provide a method of obtaining very low frequency and extra low frequency radiation spectrum from plasmas.

Another object is to provide new and improved methods and apparatus of determining the characteristics of plasmas.

A principal object of the present invention is to provide improved spectrometers for the analysis of electromagnetic spectrum.

A still further object is to provide a sharply tuned very low frequency or extra low frequency spectrometer.

Still another object is to provide a very low frequency and an extra low frequency spectrometer whose sweeping mechanisms are coordinated to provide records of received electromagnetic radiations which are coordinated in time.

Further objects and advantages will become apparent from the following detailed description taken in connection with the accompanying drawings.

In the drawings:

FIG. 1 is an end elevational view of an antenna which forms a portion of a preferred embodiment of the present invention;

FIG. 2 is a side elevational view of the antenna shown in FIG. 1;

FIG. 3 is a perspective view of a tuning mechanism which forms a portion of the preferred embodiment of the invention; and

FIG. 4 is a schematic diagram of the preferred embodiment of the invention of which the antenna illustrated in FIGS. 1 and 2 and the tuning mechanism illustrated in FIG. 3 are parts.

While this invention is susceptible of embodiment in many difi'erent forms, there is shown in the drawings and will he rein be described in detail, an embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiment illustrated. The scope of the invention will be pointed out in the appended claims.

Some of the aforementioned seven mechanisms will emit radiation only in a limited frequency band and have a spectral characteristic, whereas other mechanisms will create the white noise emission occurring over a wide band of frequency. In general, the radiation mechanisms, which emit over a wide frequency range, have a limited value for diagnostic purposes, and the mechanisms which radiate particular spectral characteristics are the most useful for establishing the detail characteristics and properties of a plasma.

Energetic plasma will emit radiation with spectral characteristics, but the data must be separated from the broad band white noise. Therefore, it is mandatory that the electromagnetic radiation be examined by means of narrow-bandwidth techniques. Spectral analyses of the emissions in the radiofrequency portion of the electromagnetic spectrum are equally as important, if not more important, than analysis of the optical portion of the spectrum. Of the seven aforementioned types of emissions, only nuclear magnetic resonance radiation is not believed to create electromagnetic radiation in the VLF (very low frequency) or ELF (extra low frequency) bandsv James A. Lasaters copending application, Ser. No. 297,802 filed July 26, 1963, now abandoned for methods and apparatus for detecting very low frequency and extra low frequency electromagnetic radiation discloses a coil-type antenna which has a quality factor (Q) in the range between 200 and 500. Since the Q of the receiving tuned circuit is dependent primarily upon the Q of the coil, the Q of the coil should be as high as possible. In order to provide for a spectrometer that will have a selectivity of approximately five cycles in the very low frequency band and as low as one cycle in the extra low frequency band, the present invention provides an improved antenna of the general type disclosed in the copending application which has a much' higher Q of approximately L300.

At the present time, spectrometers in the optical range of the spectrum and in the low-frequency and higher frequency bands of the radio spectrum are commonly used in analytical work. However, in the past it has not been possible to build spectrometers for the VLF or ELF frequency bands. Therefore, the present invention provides not only a spectrometer for these bands, but it provides a spectrometer which is specially suited for analytic work on the characteristics of plasma and plasma electromagnetic radiations.

Turning now to FIGS. 1 and 2, the apparatus forming a portion of the preferred embodiment of the invention will be described in detail.

A first group of IO ferromagnetic rods are laid parallel to each other. In like manner, a second group 11 of 10 ferromagnetic hollow rods lay parallel to each other. The rods are each 8 inches long and have an external diameter of three-eighth inch with an internal bore diameter of five thirty-seconds inch. The two groups of rods are positioned end-to-end so that they appear to be 10 hollow rods, each 16 inches in length. The groups of rods 10 and 11 are surrounded by a third group of rods 12, each of the same size. The rods are supported in an electrically nonconductive tube 15, as described in greater detail in the copending application. Each end of the tube 15 has a respective retaining disc 16 or 17 secured thereto to form a spool upon which is wound 5,000 turns of No. 18 copper wire in layers of approximately 100 turns per layer to form a coil 18. The layers are separated by 5-mil electrically nonconducting sheets of material. The resistance of the wire is approximately 31 ohms. With this method of construction, a Q of approximately 1,300 may be obtained.

The discs 16 and 17 are secured to a pair of support members 19 and 20 which are secured by a set of screws 21 which are, in turn, secured to a base comprising a horizontal member 22 and vertical members 23 and 24, all secured to each other as shown in FIGS. 1 and 2. A pair of terminals for receiving single wire shielded cables 25 and 26 are mounted on the disc 16 with shield portions 29 and 30, respectively, of the connectors connected to one end of the coil, a central terminal 27 of connector 25 connected to the other end of the coil, and a center terminal 28 of the connector 26 connected to a tap on the coil.

The lower extension of the vertical parts of the base 23 and 24 are utilized to connect the base to a tripod mount which allows the antenna to be rotated in a horizontal plane.

Referring now to FIG. 3, a pair of continuously variable tuners 31 and 32 are driven through a gear train generally indicated at 33 by an electric drive motor 34. Both tuners are multiplate, airgap condensers wherein the movable plates can be continuously rotated on shafts 35 and 36, respectively, so that the capacitance of each tuner is varied from its maximum to minimum and back to its maximum continuously when the shafts are driven by the motor 34. The tuner 31, when connected in parallel, as shown in FIG. 4, with the antenna coil 18 will form a tuned circuit whose resonance frequency is swept between 3,000 and 800 cycles per second as the capacitance of condenser 31 is swept from its minimum to its maximum value. Referring again to FIG. 4, an antenna of the type illustrated in FIGS. 1 and 2 and having a coil 18a is similarly connected across tuner 32 in order to form a tuned circuit which has a resonance frequency which is varied between 5,000 and 2,800 cycles per second as the condenser 32 is rotated from its maximum capacitance to its minimum capacitance. The gearing between shaft 35 and 36 is so designed that tuner 31 will sweep its tuned circuit from 800 cycles per second to 3,000 cycles per second while tuner 32 sweeps its tuned circuit from 2,800 to 5,000 cycles per second.

As each tuned circuit is swept through its aforementioned respective range of frequency, it produces an electrical signal which is proportional in magnitude to the magnitude of received radiation at the tuned circuits instantaneous resonant frequency as the resonant frequency is being varied.

Referring specifically to the tuned circuit composed of antenna l8 and tuner 31, an amplifier 40 is connected between the coil end terminal 29 and the tap terminal 28 by connecting a shielded cable to connector 26. A shielded cable 41 is also connected between the connector 25 and the tuner 31 to provide the connection to complete the tuned circuit as illustrated in FIG. 4. The amplifier 40 may be of any conventional design having a band-pass sufficient to cover from 800 to 3,000 cycles per second. In like manner, an amplifier having a band-pass covering the frequency of range from 2,800 through 5,000 cycles per second has its input connected between an end 290 and a tap terminal 28a of antenna coil 18a. Terminals 27a, 28a and 29a connected to coil 18a correspond to terminals 27, 28 and 29 connected to coil 18 and are connected to corresponding elements in the same manner.

The outputs of amplifier 40 and amplifier 42 are connected to a tape recorder 50 so that the amplitude of the amplified signals produced by the respective tuned circuits as they are swept to their respective frequency range is continuously recorded. The recorder 50 has all five recording channels. Another one of these channels is connected to a WWV receiver 51 so that an absolute timing reference is placed upon the tape. A fourth channel of the tape recorder is connected to an audio amplifier 52 for the purpose of putting voice and other data on the tape as it runs through the recorder 50. The amplifier 52 and the WWV receiver may be of any conventional design presently known to those skilled in the art. A pin 53 on one of the gears of the gear train 33 trips a microswitch 54 each time it completes a revolution. The microswitch 54 is connected in series with a source of electrical potential so that an electrical pulse is sent to a fifth channel of the tape recorder 50. The pin 53 and the microswitch 54 with a source of voltage constitute a sweep marker schematically shown as 55 in FIG. 4. The function of the sweep marker is to place a mark periodically on the tape when the tuners 31 and 32 have tuned their respective tuned circuits to a particular frequency in each sweep. By comparing any portion of the tape with its position relative to sweep markers, the resonant frequency of each tuned circuit at any particular time may be ascertained.

The apparatus described may be utilized for detecting and analyzing plasma radiation either in the laboratory or in the field. In the laboratory, the antennas may be set up at any suitable location close to where it is expected that plasma radiation will occur. In the field, the antenna coils 18 and should have their respective tripods (not shown) or other supporting means so positioned that the antennas may be rotated. The antennas should then be rotated to a bearing close to that from which radiation is expected. The drive motor 34, the amplifiers 40 and 42, and tape recorder 50 should be turned on. In addition, if the WWV receiver 51 or the voice and data amplifier 52 is used, these pieces of equipment should also be turned on so that the system is in a fully operational state.

frequency at which its respective antenna is receiving plasma 1 radiation, a signal will be produced by that tuned circuit which is amplified in the respective amplifier and the amplified signals are recorded by the tape recorder 50. Thus, a VLF spectrum and an ELF spectrum of a plasma radiation is obl tained simultaneously. If the system is being utilized to study a plasma which is known to be radiating, the spectrum is analyzed for a particular spectrum which is indicative of known characteristics of the plasma or it may be utilized to discover new spectrum which indicate new characteristics or new modes of presently known characteristics. Thus, characteristics of a particular plasma may be determined and new characteristics may be discovered and investigated.

The selectivity of the tuned circuits governs the quality of the spectrum analysis. Prior to the present invention, a selectivity of hundreds of cycles in the VLF and ELF bands was the best that could be obtained. With a Q of approximately 1,300 for the coils l8 and 18a which is preserved at this high level by carefully locating the taps on the coils, the tuned circuit of coil 18 and tuner 31 has a selectivity of less than five cycles over its range of frequency, and the tuned circuit of coil 18a and tuner 32 has a selectivity of less than cycles over its range of frequency. Thus, the system is a very high precision spectrometer for both the VLF and ELF bands.

The present invention may also be utilized to detect the presence of a plasma by the radiation it produces. Thus, the described apparatus may be used to continuously monitor a particular area for the occurrence of plasma radiation indicating that a plasma is in existence. Equipment may be further used to detect the existence of plasma radiation which is indicative of the mechanism which is producing the particular plasma. For example, a high-speed jet aircraft, the rocket plume of a guided missile, the ion sheath around a reentering warhead, or a nuclear detonation. Although the spectra produced by a jet aircraft on a rocket motor of a missile varies with the type of engine employed and the conditions under which it is operating, analysis of the VLF or ELF spectrum of such vehicles can be utilized to detect the vehicle, identify it as an aircraft or missile, and possibly identify the particular type of aircraft or missile. In like manner, although the spectrum produced by the ion sheaths of various bodies reentering or traveling at high speed through the atmosphere are not identical, they do produce spectra which will identify them as reentering high-speed objects producing an ion sheath and may produce a sufficiently recognizable spectra to identify the particular body creating the ion sheath. Again, in like manner, the spectra received from a nuclear detonation has features which distinguish it as such from other plasma-producing mechanisms. From the standpoint of detecting missile firings and nuclear detonations over long ranges, the VLF and ELF spectra are particularly useful because even though the plasma is located at a low altitude below the visual horizon, these electromagnetic waves will bend around the earths curvature and will have the lowest attenuation of any frequency band in the radio spectrum. lf the recorder is monitored for the occurrence of a plasma indicating such an occurrence as a missile firing or nuclear detection, either or both antennas can be rotated until maximum amplitude signals are received indicating that the pattern of the antenna is aligned with the bearing of the plasma from which the radiation is being received. This bearing can then be noted and will thus provide not only the detection of the occurrence, but the bearing of the occurrence from the detector. If two or more stations utilizing such detectors are employed, triangulation may be utilized to locate the plasma indicating a missile firing or a nuclear detonation.

Although only a preferred embodiment of the invention has been described and illustrated and preferred methods of detecting plasma-producing occurrences have been described, those skilled in the art will recognize the broad potential of the methods and apparatus herein disclosed for both analytic work in the field of plasma physics and for the detections of vehicles and explosions over long distances as well as the detection of weak plasma-producing mechanisms such as that produced by a submerged submarine.

We claim:

1. An antenna for VLF and ELF electromagnetic waves comprising:

a first multiplicity of ferromagnetic rod sections parallel to each other and having equal overall lengths,

a second multiplicity of ferromagnetic rods of equal overall lengths which are shorter than the lengths of the first multiplicity of rod sections, said second multiplicity of rods surrounding said first multiplicity of rod sections,

a coil of wire wound around said multiplicities of rods, and

a mounting means for supporting said rods and coil.

2. An antenna for extra low frequency and very low frequency electromagnetic waves comprising:

a first multiplicity of hollow ferromagnetic rod sections parallel to each other and having equal overall lengths,

a second multiplicity of hollow ferromagnetic rods of equal overall lengths which are shorter than the lengths of the first multiplicity of rod sections, said second multiplicity of rods surrounding said first multiplicity of rod sections,

a coil of wire wound around said multiplicities of rods, and

a mounting means for supporting said rods and coil.

3. A radiation spectrometer comprising:

an antenna,

a tuner connected to said antenna to form a high Q tuned circuit whose resonance frequency is capable of being varied through a selected range of frequencies below 30 kilocycles, I

a drive means for sweeping said variable tuned circuit through said range of frequencies connected to said tuner,

an amplifier connected to said antenna,

a recorder connected to said amplifier, and

a means for producing a signal actuated by said drive means and connected to said recorder.

4. In combination with the spectrometer specified in claim 3, a timing receiver connected to said recorder.

5. In combination with the spectrometer specified in claim 3, a data-receiving means connected to said recorder.

6. A radiation spectrometer comprising:

a plurality of antennas,

a tuner connected to each said antenna to form a plurality of high Q tuned circuits whose respective resonance frequencies are capable of being varied through selected ranges of frequencies below 30 kilocycles,

a drive means for sweeping said variable tuned circuits through said respective ranges of frequencies connected to said tuners,

an amplifier connected to each said antenna,

a recording means connected to each said amplifier, and

a means for producing a signal actuated by said drive means and connected to said recording means.

7. A spectrometer as specified in claim 6, wherein the recording means is a single recorder which simultaneously records signals received from said amplifiers and said signalproducing means.

8. A radiation spectrometer comprising:

a ferromagnetic core,

a wire coil wound around said core,

a variable condenser connected across said coil,

a drive means for sweeping said variable condenser through a selected range of capacitance values connected to said condenser,

an amplifier connected to said coil and condenser,

through respective selected ranges of capacitance values connected to the condenser,

an amplifier connected to each connected coil and condenser,

a recording means connected to said amplifiers, and

a means for producing a signal actuated by said drive means and connected to said recording means. 

1. An antenna for VLF and ELF electromagnetic waves comprising: a first multiplicity of ferromagnetic rod sections parallel to each other and having equal overall lengths, a second multiplicity of ferromagnetic rods of equal overall lengths which are shorter than the lengths of the first multiplicity of rod sections, said second multiplicity of rods surrounding said first multiplicity of rod sections, a coil of wire wound around said multiplicities of rods, and a mounting means for supporting said rods and coil.
 2. An antenna for extra low frequency and very low frequency electromagnetic waves comprising: a first multiplicity of hOllow ferromagnetic rod sections parallel to each other and having equal overall lengths, a second multiplicity of hollow ferromagnetic rods of equal overall lengths which are shorter than the lengths of the first multiplicity of rod sections, said second multiplicity of rods surrounding said first multiplicity of rod sections, a coil of wire wound around said multiplicities of rods, and a mounting means for supporting said rods and coil.
 3. A radiation spectrometer comprising: an antenna, a tuner connected to said antenna to form a high Q tuned circuit whose resonance frequency is capable of being varied through a selected range of frequencies below 30 kilocycles, a drive means for sweeping said variable tuned circuit through said range of frequencies connected to said tuner, an amplifier connected to said antenna, a recorder connected to said amplifier, and a means for producing a signal actuated by said drive means and connected to said recorder.
 4. In combination with the spectrometer specified in claim 3, a timing receiver connected to said recorder.
 5. In combination with the spectrometer specified in claim 3, a data-receiving means connected to said recorder.
 6. A radiation spectrometer comprising: a plurality of antennas, a tuner connected to each said antenna to form a plurality of high Q tuned circuits whose respective resonance frequencies are capable of being varied through selected ranges of frequencies below 30 kilocycles, a drive means for sweeping said variable tuned circuits through said respective ranges of frequencies connected to said tuners, an amplifier connected to each said antenna, a recording means connected to each said amplifier, and a means for producing a signal actuated by said drive means and connected to said recording means.
 7. A spectrometer as specified in claim 6, wherein the recording means is a single recorder which simultaneously records signals received from said amplifiers and said signal-producing means.
 8. A radiation spectrometer comprising: a ferromagnetic core, a wire coil wound around said core, a variable condenser connected across said coil, a drive means for sweeping said variable condenser through a selected range of capacitance values connected to said condenser, an amplifier connected to said coil and condenser, a recorder connected to said amplifier, and a means for producing a signal actuated by said drive means and connected to said recorder.
 9. A radiation spectrometer comprising: a plurality of ferromagnetic cores, a wire coil wound around each said core, a variable condenser connected across each said coil, a drive means for sweeping said variable condensers through respective selected ranges of capacitance values connected to the condenser, an amplifier connected to each connected coil and condenser, a recording means connected to said amplifiers, and a means for producing a signal actuated by said drive means and connected to said recording means. 